I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!

 

Signal averaging (numerical processing of many samples) can help.

If you do FFT on sample set what is the fundamental and first 2-3 harmonics ?

Is this AC line induced noise ? 50/60 Hz ?

Regards, Dana.

 

I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!

The data sheet for the eval ad7156 shows that within the software options is a setting for the ‘average time constant’ – setting this at a higher value may eliminate/reduce the varying output.

If you are using the USB input to power the board, make sure it is stable and not the cause of the problem – with the on board regulator having a minimum input voltage of 4V.

 

Thank you for the replies.

First i want to know is it possible to measure femto Farads accurately??

 

Thank you for the replies.

First i want to know is it possible to measure femto Farads accurately??

femto is 10^-15 and would be very difficult to measure but not impossible.

You will not be able to measure it with traditional probes. You will have to measure the effects of it in an active circuit such as an oscillator in 100GHz region.

 

I want to measure the capacitance in the form of femto farads. at present i am using eval ad7156 evaluation board. But the capacitance which i am measuring through this device is not stable.
For example i measuring 1500fF it is varying between 1480fF to 1530fF continously.

Please suggest some thing so that i can meausre fF capacitance in stable value.

Thank you!

femto is 10^-15 and would be very difficult to measure but not impossible.

You will not be able to measure it with traditional probes. You will have to measure the effects of it in an active circuit such as an oscillator in 100GHz region.

To add to MrChips, it would have to be high frequency or high impedance and almost everything will affect your measurement. Tuff Call!

 

In theory, you should be able to make one with just calculated distance between plates. But then...you probably couldn't shield it. I have heard some electric fields can not be shielded.

 

In theory, you should be able to make one with just calculated distance between plates. But then...you probably couldn't shield it. I have heard some electric fields can not be shielded.

Yes the magnetic ones, they are everywhere.

 

The only way I can think to actually do this is:
1. Set up the highest frequency source you can find, 3 GHZ is about 1PF or 1000fF into 50 ohms.
2. Make a circuit with say 10 pF, find the -3DB (about 300Mhz) point than add your cap and note the difference by readjusting for -3DB. A 1 to 2 pF cap will shift your readings by 10% to 20%. it is just the opposite way of making an accurate measurement, but it is pretty much all you have!
This all needs to be free air, point to point wiring.